In the conditioning and controlling of the temperature of molten glass flowing through a forehearth, several heating systems have been used in the past. It should be recognized that one of the important aspects of the ability to form glass articles is to provide glass to the forming machines with the glass at its best forming viscosity. With this in mind, it has been the practice in the past to melt the ingredients that form the glass in a relatively large tank termed a "melter" or "furnace" and then, in order to ensure the complete melting of all of the ingredients and to ensure complete chemical reaction to have taken place, the glass in the melter is usually fed to a chamber which is termed a "refiner". The interconnection between the furnace and the refiner is usually by way of a submerged channel. In this manner, unmelted light materials are prevented from passing into the refiner. Furthermore, reactions taking place within the glass during the melting may produce gaseous inclusions within the glass, which inclusions, if carried forward into the glass used to produce articles, might result in glass articles having defects. In the refiner, the glass is held at an elevated temperature for a period of time which is normally sufficient to permit the gaseous inclusions to expand and move upwardly to the surface of the glass. Glass from the refiner is then fed through an enclosed channel, termed a "forehearth", to the position where its extreme end will be overlying the location of forming machines such as the well-known IS machine for forming glass containers. Essentially, the forehearth serves as a glass conditioning channel through which the glass must flow and permits or provides the opportunity to control the temperature of the glass during its movement therethrough such that when the glass arrives at the remote end thereof, it will be in the condition most desirable for forming glass containers. The remote end of the forehearth is termed the "feeder" and in the case of glass container manufacturing, the feeder will deliver changes of glass termed "gobs" to a plurality of forming machine sections.
It has been the practice in the past that the forehearth temperature was controlled by the use of gas burners or fuel burners positioned along the side of the channel with the flames from the burners supplying heat to the surface of the glass in the forehearth. In conjunction with the burners, stirrers positioned within the glass would provide a system for raising the lower glass up to the surface and pushing the surface glass downwardly into the forehearth channel. All of these various systems were provided in an attempt to optimize the temperature distribution throughout the height and width of the forehearth channel. It should be understood that, in essence, the forehearth is cooling the glass at a controlled rate and that the temperature of the glass in the forehearth will be less than that found in the melter or refiner.
Most recently, the application of heat to the glass as it flows through the forehearth has been carried out by the use of electrodes extending into the glass to which current is supplied to effect heating of the glass by the well-known Joule effect.
An example of such an arrangement is disclosed in U.S. Pat. No. 4,029,488 in which the temperature of the glass is adjusted in the forehearth by providing four zones of influence to which energy is supplied through the use of immersed electrodes positioned along the side walls of the forehearth. As stated in this patent, the glass stream is subjected only to a mild Joule effect, below-surface, heating applied in a controlled manner to accomplish temperature normalization and homogeneity throughout the stream whereupon the glass is delivered to the discharge orifice at a specified temperature.
A similar system to that disclosed in the above-mentioned U.S. Pat. No. 4,029,488, is shown and described in U.S. Pat. No. 1,928,288 to Henry, wherein a system for controlling the temperature within the forehearth is used by controlling the current flow between cross-channel electrodes. Thermocouples are used to sense the temperature of the glass in three zones whereby each zone can be separately controlled in accordance with the temperature measured. In this latter patent, the temperature control is effected just prior to entry of the glass into the feeder and the two locations immediately adjacent to the feeder.
A similar configuration to that disclosed in the Henry U.S. Pat. No. 1,928,288, is the U.S. Pat. No. to Wadman, 1,905,533 which discloses the arrangement of a series of electrodes positioned along the two sides of a forehearth channel and, as shown in FIG. 4, this arrangement is generally the full length of the forehearth. In the configuration of this patent, however, the electrodes themselves are immersed in a secondary liquid rather than in the actual molten glass that flows within a centrally positioned forehearth channel extending between and, in effect, immersed within the liquid which is heated by the electrodes. The liquid which extends around and between the electrodes is a glass which acts as a resistor. Control of the temperature within the forehearth channel in Wadman is accomplished by the selection of the amount of current being fed between opposed electrodes immersed in the glass which surrounds the forehearth channel.
A further patent of interest is that of Gell U.S. Pat. No. 3,506,769, in which a forehearth channel is provided with three-phase current through sets of opposed electrodes extending through the walls of the forehearth adjacent the bottom thereof. In addition to the use of electric heat to supply heat below the surface of the glass in the forehearth, other heat exchange means are shown as being provided above the surface of the glass. These are characterized as being fuel-fed burners. Thus this patent shows the combination of electric heating and fuel heating.
The present application is concerned primarily with controlling the temperature in the forehearth by use of electricity entirely and the prevention of seeds and blisters in the glass caused by ions attacking the electrodes.